Apple has fired off a press release suggesting just how well, though: 4 million iPhone 4S units sold this weekend, across seven countries.

Those are some pretty insane numbers, considering that they’re far better than Apple’s other iPhone 4 launches. According to Mashable, it took Apple about three weeks to push 3 million iPhone 4 units out the door 18 months ago, and 74 days to reach 1 million sales of the original iPhone. Meanwhile, on the Android side of the aisle (and by way of comparison), Samsung’s Galaxy S II smartphone has sold 10 million units after being on the market for five months. The 4S has hit 40 percent of that number in three days.

It seems like a perfect storm of events have come together to make the 4S a super-hit, beyond just the regular iPhone buzz that Apple always generates. The device is launching at a strong time for upgraders from the iPhone 3GS, whose contracts should just about be run out and ready for upgrades.

Meanwhile, Verizon and Sprint users now have access to the iPhone, which has to be accounting for massive volume. Any interested users who skipped the Verizon iPhone 4 earlier this year are likely waiting in line for the 4S, and a contingent of Sprint users have been clamoring for the iPhone for years. And then there are the recent troubles BlackBerry provider Research In Motion has been suffering, namely a worldwide service outage that affected millions. Likely events such as that one drove a few more people over to the Apple camp.

Apple released a few more numbers that suggest the scope of last week’s releases. It launched iOS 5 on Tuesday last, and already Apple is reporting that 25 million devices – iPhones, iPod Touches and iPads – have been upgraded to the operating system. Another 20 million users have jumped onto iCloud, Apple’s new cloud-based service that replaces its former MobileMe service.

Pocket Gamer has a few numbers for comparison here, too, pointing out that while the latest version of Google’s Android operating system, Android 2.3 Gingerbread, has been available for about a year, only about 40 percent of Android devices have been upgraded to use it. Microsoft’s latest version of Windows Phone 7, Mango, is moving a bit faster according to unofficial statistics, reaching 30 percent of device users in just two weeks. For Apple’s part, the 25 million represents about 11 percent of the total iOS user base according to estimates, but that number also incorporates older devices that can’t be upgraded, like the iPhone 3G or later-generation iPod Touches.

Of course, Apple has a lot fewer devices and none of the fragmentation to deal with that its two rivals do. But it is interesting how quickly and deeply Apple can distribute major updates such as iOS 5. And if you were having trouble upgrading last week, it was because so many other users were trying to do the same, not because of deeper fragmentation or hardware issues.